What is the Process of Atmospheric and Vacuum Distillation?
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Understanding Atmospheric and Vacuum Distillation in Petroleum Refining
Petroleum refining stands as a cornerstone of modern industrial processes, with distillation serving as its fundamental operation.
The fundamental process in petroleum refining involves the separation of crude oil into different fractions (referred to as “distillates”) based on their boiling point ranges. This process typically includes three key stages:
- Crude Oil Pretreatment:
The removal of water and salts from crude oil. - Atmospheric Distillation:
At near atmospheric pressure, crude oil is distilled to extract straight-run fractions such as gasoline, kerosene (or jet fuel), and diesel. The bottom residue from the distillation tower is known as atmospheric residue (commonly referred to as heavy oil). - Vacuum Distillation:
Conducted at an absolute pressure of around 8 kPa, vacuum distillation is used to distill heavy fractions to produce lubricating base stocks, feedstock for cracking, or pyrolysis feedstock. The bottom residue of this process is vacuum residue.
Process Flow
The Foundation: Crude Oil Pretreatment
Before any distillation can begin, crude oil must undergo thorough pretreatment. This initial stage focuses on eliminating impurities that could compromise the entire refining process.
Crude oil pretreatment involves the use of electrochemical separation or heating and settling methods to remove water, salts, and solid impurities from the crude.
- Purpose: To prevent corrosion and scaling caused by salts (such as sodium, calcium, and magnesium chlorides) and hydrogen chloride formed from their dissociation during heating.
- Method: When using electrochemical separation, a few to several dozen parts per million (PPM) of a demulsifier and softened water are added to the crude oil. The mixture is subjected to a high-voltage electric field (field strength of 1.2–15 kV/cm) to agglomerate and settle the saltwater droplets, effectively removing salts, water, and other impurities.
Atmospheric Distillation: The First Major Separation
The atmospheric distillation process employs a sophisticated double-tower system that operates near atmospheric pressure. In this system, crude oil is heated to temperatures between 360-370°C before entering the main distillation column, which typically contains 36-48 trays. This careful temperature control allows for precise separation of different petroleum fractions.
Atmospheric Distillation
Double-Tower System:
- Initial Distillation Tower: Distills a large portion of light oils.
- Main Atmospheric Distillation Tower:
Crude oil, heated to 360–370°C, enters the atmospheric distillation column (typically 36–48 trays).- Tower Top Product: Gasoline fraction (also called naphtha).
- Side Draws:
- Line 1: Jet fuel (aviation kerosene) or kerosene fraction, treated in a stripping column using steam or a reboiler to control the light component content (measured by the flash point).
- Line 2: Light diesel fraction.
- Line 3: Heavy diesel or transformer oil fraction (used as lubricating base stock).
Vacuum Distillation: Handling Heavy Fractions
When it comes to processing heavier petroleum components, vacuum distillation proves invaluable. Operating at an absolute pressure of 2-8 kPa, this process handles fractions with boiling points between 370-535°C. This reduced pressure environment is crucial, as attempting to separate these components at atmospheric pressure would require temperatures exceeding 420°C, potentially triggering unwanted thermal cracking reactions.
- Conditions: Vacuum distillation operates at an absolute pressure of 2–8 kPa.
- Process: Atmospheric residue is heated in a vacuum furnace to 380–400°C and introduced into the vacuum distillation column.
- Products:
- Lubricating Oil Fractions: Extracted in columns with 24–26 trays.
- Fuel Oil Fractions: Extracted in columns with 15–17 trays.
- Products:
This process enables the efficient separation of crude oil into products with distinct boiling ranges, while minimizing thermal cracking and maximizing yield.
The Integration of Processes
The success of petroleum refining lies in the seamless integration of these distillation processes. Each stage builds upon the previous one, creating an efficient system that maximizes the value extracted from crude oil while minimizing waste and unwanted reactions. The careful control of temperature, pressure, and separation parameters ensures optimal product quality and yield.
Through this sophisticated combination of atmospheric and vacuum distillation, refineries can effectively separate crude oil into distinct fractions with specific boiling ranges and properties. This fundamental process continues to evolve with technological advances, maintaining its position as the backbone of the petroleum refining industry.
The precision and complexity of these distillation processes highlight the remarkable achievements in chemical engineering that make modern petroleum products possible. As refineries continue to optimize these operations, they contribute to more efficient and sustainable use of our petroleum resources.
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